The present invention relates to surface inspection apparatus and more particularly to such a system in which a laser beam is scanned across the surface to be inspected and the scattered light is analyzed to detect flaws in or particles on the surface.
It has previously been known to inspect various surfaces by scanning a laser beam across the surface and then analyzing the light scattered to determine the presence of defects or particles on this surface. For example, a system for detecting dust particles on the surface of glass reticle plates is disclosed in co-assigned U.S. Letters Pat. No. 4,402,607 issued Sept. 6, 1983 to Lance McVay and Pedro Lilienfeld. In the system disclosed in that patent, the effects of semi-specular reflection and characteristic regular scattering caused by the pattern on the reticle were minimized, though not eliminated, by the use of a scanning beam which approached the surface at an acute angle and by utilizing only the back-scattered components of the light scattered from the surface being inspected.
While the present invention was devised as an improvement to the system shown in the '607 patent, it has broader application and in many ways overcomes limitations which had characterized this and other prior art systems.
Among the several objects of the present invention may be noted the provision of a novel surface inspection apparatus utilizing a scanned light beam; the provision of surface inspection apparatus having improved sensitivity for defects relative to regular pattern noise in the system; the provision of such apparatus which permits a relatively wide range of scanning and detection angles; the provision of such an apparatus which permits detection and distinguishing of defects of various types; the provision of such apparatus which is highly accurate, which is highly reliable, and which is of relatively simple and inexpensive construction. Other objects and features are in part apparent and in part pointed out hereinafter.